The present invention relates to a twin-wire former in a paper machine which pinches the stock between horizontal running portions of the upper and lower wire loops to dewater the stock.
In a twin-wire former of a conventional paper machine, each of two wires respectively form a loop, and while the stock is pinched between the wires, the stock is dewatered by various dewatering devices so that a fiber mat is gradually grown and a web is formed.
In FIGS. 6 and 7, two typical twin-wire formers are shown. In the twin-wire former shown in FIG. 6, a top wire 2' is engaged with a bottom wire 1' corresponding to a conventional long net used as a Fourdrinier wire so that upward dewatering can be additionally performed, and this type of former is referred to as an on-top-former or a hybrid-former. Stock 14 is injected from a head box 5' to a location between a breast roll 3 and a forming board 6'. While conveyed on the forming board 6', a foil 11, and a vacuum foil box 12, the stock 14 is dewatered downward and a mat is formed from the side of the stock facing downward. After that, the stock 14 is pinched by the two wires in a gap 13' which is formed by the top wire 2' and the bottom wire 1' on a forming shoe 7', and then the stock 14 is dewatered mainly upward by tension caused by the two wires and pulse pressure generated in a portion of the forming shoe 7'.
An advantage of the aforementioned former is that since the former is provided with a Fourdrinier wire type of preforming zone before the stock 14 is pinched between the top wire loop 2' and the bottom wire loop 1', a paper of high quality can be made which has high strength in a thickness direction so that a degree of orientation (a ratio of longitudinal to lateral tensile strength) is low. On the other had, the aforementioned former has the following drawbacks. The stock 14 in the preforming zone has a free surface. Accordingly, when the stock 14 is conveyed at high speed, air resistance and an agitation effect caused by the foil become excessive, so that the surface of the stock is disturbed and jumping occurs. Therefore, the operation becomes difficult, and a deterioration in paper quality such as degradation in the formation and increase in the air permeability are caused.
In the former shown in FIG. 7, a bottom wire 1" and a top wire 2" are engaged with each other immediately after a breast roll 3 and a forming roll 4 so that a wedge-shaped gap 13 is formed. Accordingly, this type of former is referred to as a gap former or a true twin-wire former because it is not provided with a Fourdrinier wire type preforming zone. Stock 14 injected from a head box 5 is pinched by the two wires in the gap 13 located immediately after the breast roll 3 and the forming roll 4. Then, the stock 14 is dewatered simultaneously upward and downward due to the squeezing effect by the wire tension and pulse pressure acting on the stock 14 in a forming shoe 7" arranged after the gap 13.
An advantage of this former is that pulse-like dewatering pressure acts on the stock from when the stock concentration is low, so that a good mat can be formed. Further, this former is characterized in that the operation can be conducted at high speed since the stock injected from the head box 5 is immediately pinched by the two wires; and the stock does not flow laterally in the preforming zone, so that the angle of orientation of the stock injected from the head box can be maintained to obtain a paper in which the angle of orientation in the width direction is uniform. On the other hand, in this former, when the thickness of stock is large (that is, when the concentration is low), the stock is pinched by the two wires, so that the degree of orientation (the ratio of longitudinal to lateral tensile strength) becomes high by the pressure generated when the stock is pinched. Further, this former has a drawback in that since the stock is dewatered toward both sides by pulse-like pressure applied during an initial stage of dewatering, the ratio of dewatering to the upper and lower sides becomes approximately 50/50, so that the binding strength in a middle layer of the mat is lowered, and the strength in the thickness direction of the paper becomes low.
As explained above, the twin wire former which has been used as a mainstream of formers nowadays has merits and demerits. Therefore the type of a former is selected in accordance with the kind of paper and the characteristics thereof desired.
That is, in the case of newspaper, the formation and speed of fabrication are important, so that a gap former is utilized. A hybrid former is mainly used in the case of middle and high class paper such as information paper (a PPC paper) in which high lateral rigidity is required (a low ratio of tensile strength is required) in order to improve the running property when the paper is used in a copier, and in which a low curling property (a difference in nature between its front and rear surfaces is small, and an angle of orientation is small) is required, and such as coating paper in which a high strength in the thickness direction is required in order to mitigate a problem caused by blisters.
Especially in the latter case, the required quality of the paper, to which priority is given, is different case by case, and at the same time the formation, which is an essential quality of paper, and high speed, i.e. high productivity is manufacture, are required. Consequently, when these various kinds of papers are made by one former, a compromise between the various qualities required for the different types of paper must necessarily occur.